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Modern society depends on energy for its existence. Any symptom of an energy shortage—rolling blackouts of electrical power, gasoline shortages, or big increases in the cost of natural gas—are enough to shake people’s confidence and roil the markets. Energy is very much a chemical topic. Nearly all of the energy on which we depend is derived from chemical reactions, such as the combustion of fossil fuels, the chemical reactions occurring in batteries, or the formation of biomass through photosynthesis. Think for a moment about some of the chemical processes that we encounter in the course of a typical day: We eat foods to produce the energy needed to maintain our biological functions. We burn fossil fuels (coal, petroleum, natural gas) to produce much of the energy that powers our homes and offices and that moves us from place to place by automobile, plane, or train. We listen to tunes on battery-powered MP3 players.

The relationship between chemical change and energy shows up in various ways. Chemical reactions involving foods and fuels release energy. By contrast, the splitting of water into hydrogen and oxygen requires an input of electrical energy. Similarly, the chemical process we call photosynthesis, which occurs in plant leaves, converts one form of energy, radiant energy from the Sun, to chemical energy. Chemical processes can do more than simply generate heat; they can do work, such as turning an automobile starter, powering a drill, and so on. What we get from all this is that chemical change generally involves energy. If we are to properly understand chemistry, we must also understand the energy changes that accompany chemical change.

The study of energy and its transformations is known as thermodynamics (Greek: thérme-, “heat”; dy’ namis, “power”). This area of study began during the Industrial Revolution as the relationships among heat, work, and the energy content of fuels were studied in an effort to maximize the performance of steam engines. Today thermodynamics is enormously important in all areas of science and engineering, as we will see throughout this text. In the last couple of chapters we have examined chemical reactions and their stoichiometry. In this chapter we will examine the relationships between chemical reactions and energy changes involving heat. This aspect of thermodynamics is called thermochemistry. We will discuss in detail other aspects of thermodynamics in Chapter 19.

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